The present invention relates to an air-fuel ratio control apparatus for an engine that performs air-fuel ratio learning control.
To equalize the air-fuel ratio of air-fuel mixture combusted in the cylinders of the engine with a target air-fuel ratio, the amount of air combusted in the cylinders (in-cylinder air amount) may be obtained, and the fuel supply amount may be determined such that the ratio of the fuel supply amount to the in-cylinder air amount is equalized with the target air-fuel ratio. However, the output property of an air flowmeter used to calculate the in-cylinder air amount and the injection property of an injector that injects fuel differ among individuals and change over time. Thus, if the fuel supply amount is simply determined in accordance with the in-cylinder air amount calculated from the detection result of the air flowmeter, the air-fuel ratio may vary with respect to the target air-fuel ratio. The variation of the air-fuel ratio is corrected by performing air-fuel ratio feedback control that corrects the fuel supply amount in accordance with the deviation of the air-fuel ratio from the target air-fuel ratio.
Air-fuel ratio learning control is further performed that learns the variation of the air-fuel ratio as an air-fuel ratio learning value from the result of the air-fuel ratio feedback control. The air-fuel ratio learning value that has been learned is previously reflected in the fuel supply amount so that the responsiveness of the air-fuel ratio feedback control is improved. The variation tendency of the air-fuel ratio varies even in the same engine depending on the operating region of the engine. Thus, learning of the air-fuel ratio learning value is desirably performed separately for each operating region. For example, the air-fuel ratio control apparatus disclosed in Japanese Laid-Open Patent Publication No. 2006-258037 performs, in an engine that includes two types of injectors for port injection and for direct injection, learning of the air-fuel ratio learning value for learning regions divided in accordance with the type of the injector, warm operation/cold operation of the engine, and the intake air amount.
However, segmentation of the operating regions (learning regions) for separate learning reduces the time spent for each learning region and reduces the learning opportunities of each learning region. Thus, the time required for completing learning of all the learning regions is exponentially increased in accordance with the increase in the number of the learning regions.
The air-fuel ratio control apparatus disclosed in Japanese Laid-Open Patent Publication No. 2006-258037 calculates, based on the air-fuel ratio learning value of the learning region in which learning has been completed, the air-fuel ratio learning values of other learning regions in which only the intake air amount differs by, for example, a linear interpolation. This advances learning of the air-fuel ratio learning values in the learning regions with small learning opportunities. The air-fuel ratio control apparatus disclosed in the Japanese Laid-Open Patent Publication No. 2005-105978 stores the variation tendency in the air-fuel ratio of each learning region on an air-fuel ratio learning map and calculates the air-fuel ratio learning values of other learning regions from the air-fuel ratio learning value of the learning region in which learning has been completed using the air-fuel ratio learning map.
In the air-fuel ratio control apparatuses of the above publications, the learned result of the learning region in which learning has been completed is reflected in the air-fuel ratio learning values of other learning regions to improve the efficiency in learning the air-fuel ratio. In the air-fuel ratio control apparatuses of the above publications, after learning of the air-fuel ratio learning value has been completed in any of the learning regions, learning in other learning regions is promoted. However, learning needs to be advanced separately in each learning region until learning in other learning regions is promoted. Thus, when the air-fuel ratio learning value needs to be learned from the beginning in all the learning regions, such as after clearing the battery, relatively long time is taken until the air-fuel ratio is learned efficiently.